For data transmission on a standard voice grade telephone line a modem is employed which is operative to modulate data in a form suitable for transmission along the telephone line, and to demodulate data received from the telephone line to reproduce the data transmitted from a remote site. The modem requires a power supply for energizing its electronic circuits and this power supply is usually operative from an AC power outlet or a battery source.
In many instances an AC power source is not conveniently available, and, in the case of a battery source, the modem may be at a location where battery monitoring or replacement is inconvenient. In addition, the power supply adds to the expense and size of a modem. It would be advantageous to provide power for operation of the modem circuits from the telephone line itself to thereby dispense with the need for any separate modem power supply or access to a separate power source.
The connection of modems to the telephone network is governed by FCC and Bell System regulations and specifications which set forth stringent engineering specifications which must be met for certification of such apparatus for connection to the telephone network. Meeting such specifications by circuits powered only from the telephone line becomes a difficult task, since the operating parameters and circuit configuration of the modem circuits at the extremely low power levels available from the telephone line are rather critical.
In U.S. Pat. No. 3,539,727 a data receiver is described which has a power supply energized by the telephone line and isolated from the line. This patent does not disclose a fully operational modem powered by the telephone line. The power supply includes a regulator circuit which provides DC power derived from the telephone line to the receiver circuits. Frequency shift data signals from the line are applied to a locked oscillator which provides limited AC signals of a frequency which is locked to the received data signal frequency. A discriminator provides recovered data from the limited signal and operates an output switch which produces data signal voltages as an output. The limited AC signal is also applied as an input to a converter circuit which produces DC power for the output switch.